In smoke detectors of the reflected light type, in which a photo-responsive device is used to receive light from smoke particles illuminated by a light source, one of the major problems has been that of providing a light source which is capable of operating over a long period of time without failure. For this purpose, light emitting diodes have recently been utilized.
However, commercially available light emitting diodes have, at their rated current, insufficient light output to function as an effective smoke detector. However, it has been found that such a diode will produce light output adequate for smoke detection purposes if it is operated at a current considerably higher than the rated current specified by the manufacturer, but its life is so short at this higher current as to make its use in a commercial smoke detector impractical.
However, I have found that if the light emitting diode is energized at the higher current in short pulses, its light output and service life will be adequate for a continuously operating smoke detector.
A detector utilizing light emitting diodes in this manner is disclosed in U.S. Pat. No. 3,946,241 issued to me on Mar. 23, 1976. In the detector disclosed therein, the pulse to the light emitting diode has a duration of about 20 micro seconds, with the repetition rate being 1 pulse every 2 seconds. The detector described therein is designed to produce an alarm only if smoke is detected on two consecutive pulses.
However, it has been found desirable in some cases to increase the degree of immunity from false alarms, to require the detection of smoke by 4 or more pulses to produce an alarm, and it has also been found desirable to reduce the pulse repetition rate to, for example, 5 seconds, to increase the life of the light-emitting diode. However, the combination of these two modifications would result in an alarm response time of 15 seconds, which is an unacceptable length of time.
It has been suggested that on the detection of smoke by a pulse, the repetition rate could be increased, so that the required number of output pulses to produce the alarm would be produced in a shorter period of time. However, if there are no subsequent output pulses (such as when the first pulse is a result of a spurious response), the pulse rate would nevertheless continue at the high rate. This not only reduces the life of the light-emitting diode, but also increases the possibility of another false alarm being received during the period of increased pulse rate.